Switching power supply devices having a power-factor improvement converter are used in electronic equipments such as AC adapters, devices and equipments for office automation, and commercial-off-the-shelf equipments. For this reason, the switching power supply devices are adapted to harmonic current restraint (IEC/EN 61000-3-2), and harmonic restraint guidelines for electrical household appliances and multipurpose products. Further, in recent years, the efficiency of the switching power supply devices is desired in order to cope with the miniaturization and the energy saving of electronic equipments.
One example of step-up chopper circuits based on the harmonic current restraint is a device disclosed in Japanese Patent Application Laid-Open No. 5-111246 (1993). In FIG. 1, a power-factor improvement converter includes a rectifying circuit 2 using a diode bridge, a switching element 4 turned on/off by a control circuit 8, and a step-up reactor 3. While the step-up chopper circuit is making a peak current of the step-up reactor 3 track an input voltage, the switching element 4 is turned on/off so that an output voltage becomes constant. The step-up chopper circuit conforms to the harmonic current restraint by performing the switching operation in such a manner.